The present invention relates to a heat transfer tube of boiling type heat exhanger, having a high heat transfer coefficient and easy to manufacture, as well as to the method of producing the same.
In general, heat transfer tubes of boiling type heat exchanger have surface conditions which allow residual bubbles on the heat transfer surface, and are adapted to be dipped in liquids such as from, water, liquefied nitrogen and liquefied oxygen, so as to effectively heat these liquids to make them boil.
Various types of the boiling type heat transfer tubes have been developed up to now. These pipes are, for example, (I) the heat transfer tube having a porous layer formed on its surface by sintering metallic powders, and (II) the heat transfer tube having annular or spiral cavities communicating the outside through gaps, formed by bending the top ends of a number of fins, which have been made even previously on the tube surface, in the longitudinal direction of the tube.
However, the heat transfer tube of the type (I) above, relying upon the sintering, is disadvantageous in that the tube is likely to be softened to reduce its strength during the sintering process and that the manufacturing cost is relatively high.
At the same time, the heat transfer tube of the type (II) above having a number of bent fins has such a drawback that the fins, which have to be formed to have a considerably large height, can hardly be formed on the surface of tube of hard materials such as stainless steel, and exhibit poor bending workability.
Under these circumstances, the present invention aims at providing, as a fruit of an intense study, a heat transfer tube for use in boiling type heat exchangers, having an improved heat transfer characteristic owing to provision of cavities on the tube surface for forming residual bubbles which promotes the nucleous boiling and which can easily be formed even on the tube of relatively hard material, as well as a method of producing the heat transfer tube.
More specifically, the present invention is concerned with a heat transfer tube for use in boiling type heat exchangers, having annular or spiral cavities formed on its surface, the cavities communicating the outside space through narrow gaps extending in the longitudinal direction of the cavities.
At the same time, the present invention is concerned with a method of producing a heat transfer tube for use in boiling type heat exchangers, comprising the steps of forming a number of fins on the tube surface by means of fin-forming tool, and compressing and forming the top ends of the fins so as to form annular or spiral grooves communicating the outside space through narrow gaps.
Another object of the invention is to provide a heat transfer tube for use in boiling type heat exchangers in which fins having a large heat transfer surface are formed, in addition to the above stated cavities for forming bubbles to promote the nucleous boiling, so as to improve the heat transfer performance.
To this end, the present invention is concerned with a heat transfer tube for use in boiling type heat exchangers having a number of annular or spiral fins formed on its surface, the cavities communicating the outside space through narrow gaps formed to extend in the direction of these cavities, and annular or spiral fins disposed to project to the space between adjacent gaps.
In another aspect, the invention is concerned with a method of producing a heat transfer tube for use in boiling type heat exchanger comprising, forming a number of thick-walled annular or spiral fins on the tube surface by means of a fin-forming tool, and compressing and forming the shoulder portions of adjacent thick-walled fins by means of a single or multiple staged compression-forming disk tool having a diameter smaller than that of the fin-forming tool and a breadth smaller than the pitch of the fins, so as to form at the bottoms of the thick-walled fins cavities which communicate with the outside space through narrow gaps, as well as annular or spiral projecting fins between the adjacent gaps on the tube surface.
Conventionally, there have been proposed various surface conditions for promoting and enhancing the heating and boiling characteristics. However, these conventional mechanisms involve problems or shortcomings in relation with the heat transfer performance, difficulty in manufacturing, production cost and so forth, although they may have certain advantages.
For instance, the aforementioned heat transfer tube having a porous surface layer formed by sintering of metallic powders is likely to deteriorate in heat transfer peformance due to the clogging of the cavities. At the same time, in the heat transfer tube having a surface condition consisting of a number of fine cavities and openings through which the cavities communicate with the outside space, the cavities are oriented unindirectionally, e.g. only in the circumferential direction of the tube, so that it is difficult and costly to improve the heat transfer performance through making the pitch of the cavities small.
Also, as to the production of the heat transfer tube, almost no tube has been proposed, which can be produced by only one step and which can be handled, when installed in a heat exchanger, in such a simple manner as ordinary smooth tubes or low fin tubes.
Under these circumstances, the present invention provides as its still another object, a heat transfer tube in which the boiling performance at the outside of the tube is improved by provision of such a surface condition as would promote the boiling not only in the circumferential direction but in axial direction of the tube as well.
Thus, the present invention is concerned also with a heat transfer tube and the method of producing the same, comprising annular or spiral cavities formed beneath the tube surface to extend in the circumferential direction of the tube and disposed at a desired pitch in the axial of the cross-section of the cavities irregularly changing in the longitudinal direction thereof, the cavities having normally irregularly shaped restricted gaps or openings through which these cavities communicate with the outside space, the gaps or openings extending along the cavities continuously or discontinuously, and small cavities formed under the outer surface to extend in the axial direction of the tube at a desired circumferential pitch so as to connect adjacent gaps to each other or adjacent openings to each other, the small cavities also having gaps or openings through which they communicate with the outside space. The small cavities and their gaps or openings may be in the form of small grooves having convexities and concavities in its both sides and bottom, as occasion demands.
In carrying out the invention, for achieving the objects of the invention more effectively, it is useful to provide fins protruding with their longitudes extending in the circumferential direction, at the centers of the portions between adjacent first-mentioned circumferentially extending annular or spiral cavities and between adjacent second-mentioned axially extending cavities or grooves, and to effect a knurling over the entire surface of the tube excepting the protruding fins.